This invention relates to elastomeric compounds that exhibit an exceptional combination of temperature and fatigue resistance and includes, in particular, an elastomeric composition comprising mercaptan modified polychloroprene with specially selected particulate filler as well as preferably certain other ingredients, which upon cure, yields an article exhibiting advantageously both thermal dimensional stability, particularly evidenced by compression set, as well as high original and aged fatigue resistance along with other desirable properties making it admirably suited for use in preparation of automotive suspension components such as bushings.
Elastomeric componds are used in many dynamic, fatigue-producing applications, such as automotive suspension components. Conventionally cured (i.e., high sulfur) natural rubber compounds are usually employed in such applications because of their excellent fatigue properties despite their less desirable resistance to thermal and oxidative degradation. When thermal stability also becomes a component requirement, substitutes for these traditional materials may become necessary. In the case of suspension bushings, this requirement could arise as a result of increased engine compartment temperatures, or proximity to exhaust components.
Polychloroprene is an elastomer showing good potential for replacing natural rubber (NR) in high temperature dynamic applications. It is known to have better resistance to heat and oxidation than NR and has nearly the same resiliency. It might also be expected to have good fatigue properties since it crystallizes upon strain, and strain crystallization is known to improve fatique life. Moreover, it is not as expensive as other high temperature elastomers such as epichlorohydrin and polyacrylate.
Compounding polychloroprene to exhibit highly desirable properties including desirable original and aged fatigue as well as thermal stability evidenced by low compression set, however, is difficult. Typically, as compression set is desirably lowered through compounding, properties generally associated with fatigue resistance as tear strength tend to erode. On the other hand, as tear strength is desirably increased through compounding, properties associated with thermal stability as compression set tend to erode. For example, mercaptan modified polychloroprene compounds typically show low compression set but, also, low tear resistance using conventional thiourea cure with carbon black. Polychloroprene rubbers which are sulfur modified exhibit under similar compounding high tear resistance but, also, undesirably high compression set. Moreover, replacement of certain cyclic thioureas with alkyl substituted thioureas for cure, while providing lowered compression set, undesirably reduces tear resistance.
Thus, it would appear that one would normally be left with but a tradeoff between such important properties as compression set and tear resistance and, consequently, the discovery that both of these properties can be brought to quite desirable levels is of notable value.
Essential ingredients of compounds of this invention are advantageously commercially available and others have suggested their use in certain contexts. For example, silicas have been suggested for improving certain heat aged properties as compared to carbon black (see "Reinforcing Silicas and Silicates", Wagner, Rubber Chemistry and Technology; Vol. 49, 703, 1976 particularly 751, 766 and 767 and "Filler Effect on The Heat Stability of Vulcanized Elastomeric Compositions", Fetterman, Rubber Chemistry and Technology vol. 46, 927, 1973). Further, use of certain carbon blacks, e.g. large particle size and high structure has been reported as desirable in certain compositions for improvement in fatigue properties (see, Dizon et al, Rubber Chemistry and Technology, 47, 231 (1974). None of these disclosures, however, have shown a means to provide high tear strength along with low compression set in polychloroprene rubber compounds so as to make them especially suitable for high temperature, dynamic applications as automotive suspension bushings.